Plasma accelerator



Feb. 23, 1965 P. WOOD ETAL PLASMA ACCELERATOR 2 Sheets-Sheet l FiledMarch 7, 1962 INVENTORS GEORGE R W0 ARLEN F. CAR

OD TER ADOLF BUSEMANN Feb. 23, 1965 G. P. WOOD ETAL 3,171,060

PLASMA ACCELERATOR Filed March 7, 1962 2 Sheets-Sheet 2 INVENTORS GEORGEP. WOOD ARLEN F. CARTER ADOLF BUSEMANN ja BY United States Patent3,171,060 fLASMA ACCELERATGR George P. Wood, Yorktown, Arlen F. Qarter,Newport News, and Adolf Busemann, Hampton, Va., assignors to the UnitedStates of America as represented by the Administrator of the NationalAeronautics and Space Administration Filed Mar. 7, 1962, Ser. No.178,213

Claims. (til. 315111) (Granted under Title 35, US. Code (1952), sec.266) The invention described herein may be manufactured and used by orfor the Government of the United States of America for governmentalpurposes without the payment of any royalties thereon or therefor.

This invention relates generally to a plasma accelerator, and moreparticularly to a steady fiow, linear, direct current, electromagnetic,crossed-field accelerator.

An apparatus for accelerating an ionized gaseous body, or plasma, to thestate required by space age applied science and laboratoryexperimentation, has been the object of intensive research. A suitableaccelerator has many important, potential applications, including use asa source of high-speed flow for aerodynamic testing, as a source ofhigh-speed plasma for research in magnetoplasmadynamics, and as apropulsive system for space vehicles. An especially pertinent area ofinterest in the ultilization of a steady flow accelerator lies in thelaboratory reproduction or simulation, for test and study, of as many aspossible of the atmospheric reentry conditions of velocity,

temperature, and density, to which space vehicles are exposed.

It has previously been proposed to utilize the prior art electrostaticforce principle, as in ion guns and ion engines, in the design of thedesired accelerator. However, it has been found that the density ofcharged particles produced in a device of this type is many orders ofmagnitude too small for the uses contemplated. It has been furthersuggested that, if a beam of ions ofthe necessary density cannot beproduced in this manner, the required density could be obtained byputting a relatively small percentage of ions in some gas, acceleratingthese charged particles electrostatically, and letting them collide withand thereby accelerate the neutral molecules of gas. However, thisoperation is also found to be unworkable for any reasonable density ofgas, such as that existing at 100,000 feet, because of the space chargeeffect. The space charge due to the ion density required in theaccelerating section is estimated to be in the billion-volt class.

The obvious way to overcome space charge effects is to build upessentially equal number densities of positive ions and negativeelectrons in the accelerator; but if this condition is had initially,application of an electric field to the accelerator will separate theoppositely charged particles until a suificient space charge is againbuilt up to prevent further separation of the particles, and no netincrease in momentum can be obtained.

Accordingly, it is an object of the present invention to provide a novelplasma accelerator.

Another object of the instant invention is to provied a high-densityplasma accelerator capable of steady flow operation.

A further object of this invention is to provide a plasma acceleratorwherein neutralization of positive ions through contact with theaccelerator wall is largely eliminated.

The foregoing and other significant objects are attained in the instantinvention by the provision of an accelerating channel having crossedelectric and magnetic fields. The fields are oriented normally to eachother, and the charged particles of the plasma flowing through thechannel are accelerated by a force calculable from the Lorentz equation.In addition to applying the electric field in a plane normal to that ofthe magnetic field, the electric field is also slanted in an axial,downstream direction so as to cause the positive ions of the plasma toflow nearly paral let to the channel axis; thereby reducing neutralizingcontact of the ions with the channel wall. The slanted electric field isprovided by a plurality of axially spaced, segmented electrodes whichcomprise opposing channel walls. The cathodes of the segmentedelectrodes also serve as thermionic emitters which, when heated by theplasma flow, emit an electron current. The electron current passesthrough the plasma flow and is collected by the segmented anodes of theopposite channel wall. The driving force on the plasma is dependent uponboth the density of the electron current passing therethrough, and theinduction of the imposed magnetic field.

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily apparent as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings wherein:

FIG. 1 is a vertical sectional view of the accelerator, taken along aplane normal to the channel longitudinal axis;

FIG. 2 is a longitudinal sectional view of the accelerator, taken alonglines 22 of FIG. 1;

FIG. 3 is a schematic wiring diagram illustrating a manner of connectingthe plurality of electrodes of the instant invention to unidirectionalsources of potential, to pro vide both a vertical and a horizontalpotential gradient; and

FIG. 4 is a schematic diagram illustrating the orientation of theelectro-magnetic field relative to the accelerator channel.

Referring now to the drawings, wherein like reference numerals designateidentical or corresponding parts throughout the several views, theaccelerator 11 of the present invention is illustrated, in FIGS. 1 and2, as comprising a plurality of axially spaced pairs of electrodes12-13, positioned so as to form an axial flow channel 14 between theconfronting surfaces thereof. It will be noted that the electrode 12serves as the positive electrode, or anode, and the electrode 13 as thenegative electrode, or cathode. It will also be obvious that the anode12 is much thicker than the cathode 13; the cathode 13 being mountedupon a conductive element 16, the combined thickness of cathode 13 andmounting element 16 approximating that of anode 12. This thickness ofmaterial is desirable to enable the electrical connections to theelectrodes to be made at relatively cool locations. A satisfactoryhigh-temperature material which may be utilized for the anode 12 and themounting member 16 is graphite. The cathode material 13 must be selectedwith regard both to its melting point and to its thermal emission rate.Several cathode materials have been suitably employed; three of suchmaterials being: lanthanum hexaboride, pure tungsten, and 2 percentthoriated tungsten. The 2 percent thoriated tungsten has been found tobest combine the properties of high melting point and electronemissivity. It may be observed that the accelerator 11, as shown,consists of seven pair of electrodes. However, the number of electrodepairs may be more or less than the seven shown, the number beingdependent upon the degree of electric field uniformity desired, and uponthe length of acelerator desired.

The plurality of electrode segments 12, 13 are separated by elongatedrectangular insulating strips 17; the strips forming the sidewalls ofchannel 14. An insulating spacer member 18, FIG. 2, is positionedbetween each of the axially spaced electrodes 12, 13. A preferredmaterial assembly and secured together.

block.

, channel.

for the insulating members 17, 18, possessing excellent high temperatureinsulating properties, is boron nitride.

Electrical contact to the electrodes 12, 13 is made through individualL-shaped brass plates 19; the electrical contacts 19 being pressed intoengagement with the outer surfaces of anode 12 and the cathode mountingmember 16. Pressure is applied to the electrical contacts and electrodeassembly through screws 21, the screws being insulated from electricalcontact plates 19 by members 22. The screws 21 are threaded throughcantilevered arms 23, which extend laterally from upper and lower wallmembers 24, 26, to exert the desired pressure.

The electrode assembly is housed in a nonmagnetic, stainless-steelcasing consisting of upper and lower wall members 24, 26, and side walls27, 28. A liner 29 of insulating material, such as boron nitride, ispositioned between the side walls and the electrodes to preventshorting. The ends of the electrode assembly are enclosed by relativelythick insulating members 31, 32, FIG. 2. The electrode assembly and wallmembers are secured together into an integral unit by clamps or bands,not shown,

.fastened to the outer, metallic wall members.

Alternatively, it is contemplated that the accelerator casing may beformed from a cast ceramic block; rather than individual wall memberssurrounding the electrode Individual holes to receive the electrodeswould then be cut in one wall of the block, and a flow channel formedthrough the ceramic If desired, the emitting cathodes can be mounted ona movable pedestal, for movement away from the flow This constructionenables the emitting wall to be lowered away from the accelerator topermit activating of the thermionic emitters by heating means other thanthe plasma fiow, without heating of the whole accelerator durlng theperiod required to activate the emitters. The electrical circuit foraccelerator 11 is schematically illustrated in FIG. 3. As shown, avertical electric field is established between each anode 12 and cathode13 by an individual direct current potential source 33. An additionalpotential source 34 is arranged between each axially spaced electrodepair 12, 13, to set up an axial or horizontal electric field in thedownstream direction.

'The net effect of these two potential gradients is to create a slantedelectric field which accelerates and directs, in conjunction with animposed magnetic field, the charged particles supplied to flow channel14.

The accelerator magnetic field is applied by an electromagnet; the poles36, 37 of which are shown in FIG. 1. The magentic field is appliedacross channel 14, perpendicular to the slanted electric field and tothe channel axis.

was produced in the gap. The flow channel was 1 cm.

square, and the accelerator was 3% inches long, with seven electrodes Ainch in length separated by A; inch insulators made of boron nitride.The cathodes 13 were made from thoriated tungsten, inch thick, and theanodes 12 from graphite, 1 /2 inches thick. The electrode assembly washoused in nonmagnetic stainless-steel Walls, with a inch thick lining ofboron nitride between the electrodes and the walls. A potential gradientof 48 volts/cm. in a vertical direction and 4 volts/cm. in an axialdirection was applied to the accelerator. This gave a current densityacross the flow channel of 16 amperes/cm.'-. Acceleration of the plasmawas verified by Pitot tube measurement of the change in stagnationpressure at the accelerator exit, and by static pressure measurements atthe entrance and exit of the accelerator channel. The above parametersare merely illustrative of one embodiment of the instant invention; andthe enlargement or reduction in scale of accelerator physical dimensionsand field strengths, or the substitution of other suitable materials forthose employed, are contemplated by the inventors thereof.

The principle of operation of the disclosed accelerator 11 is based onthe Lorentz equation, F=j B. The term F of the equation may be definedas the driving force per unit volume exerted on the plasma to beaccelerated; j as the density of electron current flow through theplasma; and B the magnetic induction of the accelerator field. Theelectron current flow through the plasma is produced by the electricfield created between pairs of electrodes 12, 13. Electrons are emittedfrom cathodes 13, pass through the plasma, and are collected by anodes12. The magnetic field is set up perpendicular to both the direction ofplasma flow and of electron current flow by poles 36, 37. The chargedparticles of the plasma introduced into the crossed-fields of theaccelerator 11 experience a driving force, normal to both the magneticand the electric fields, which accelerates the particles in an axial,downstream direction. The positive ions of the plasma, in turn, collidewith and propel neutral gaseous molecules along the accelerator channel.

In FIG. 4, the various factors set forth above are shown oriented withrespect to the accelerator flow channel 14. The magnetic field B isapplied across the channel perpendicular to and into the paper. Theelectric field E is applied across channel perpendicular to the magneticfield, but slanted with respect to the channel longitudinal axis.Electric field E produces an electric current whose density is j, andthe plasma experiences an accelerating force j B per unit volume.

To provide a clearer understanding of the instant invention, theoperation of the disclosed plasma accelerator will now be described. Aplasma flow is introduced into the left hand side, FIG. 2, ofaccelerator 11. The source of the plasma may be a plasma producingapparatus as is described in the co-pending application of Arlen F.Carter, .Serial No. 178,215, filed March 7, 1962, NASA Case No.

147. The cross electrical and magnetic fields have been activated priorto introduction of the plasma into channel 14. The plasma upon entryinto channel 14, at a temperature of approximately 5,500 K., heats upthe cathodes 13 to a temperature at which thermionic emission takesplace. The electron current emitted from the cathodes 13 passes throughthe plasma to be accelerated, and is collected by anodes 12. The chargedparticles of the plasma flow experience a driving force, calculable fromthe Lorentz equation set forth above, which accelerates the particlesalong downstream, cycloidal paths. The horizontal, or axial, potentialgradient set up between the axially spaced electrode pairs 12, 13,serves to slant the electric field with respect to the accelerator axis;the electric field thereby being tailored to direct the Lorentz forcesubstantially parallel to the channel axis. Therefore, the slantedelectric field produced by the axially spaced, segmented electrodes,prevents loss of positive ions through neutralizing collisions with thechannel walls by directing the ions away from the walls, in an axial,downstream direction. The positive ions, being accelerated throughchannel 14 by the crossed fields, in turn propel neutral gaseousmolecules along the length of the accelerator channel through collisionprocesses. The accelerated plasma then emerges from the right hand sideof the accelerator, FIG. 2, for utilization in the various test,experimental, and propulsion applications previously set forth.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. A plasma accelerator comprising: a plurality of electrode pairsspaced along a linear axis, a plasma flow channel defined by theconfronting surfaces of the electrodes of each of said pairs, oneelectrode of each electrode pair comprising a cathode adapted to beheated to its electron emission point by the plasma flow through thechannel, an electrical circuit interconnecting said plurality ofelectrode pairs, said circuit being adapted to provide an electric fieldacross said channel directed at a slant angle to said linear axiswhereby the positively charged particles of the plasma are directedalong the channel axis to prevent neutralizing collisions with thechannel walls, and means for providing a magnetic field across saidchannel directed perpendicular to said electric field, said means beingpositioned on opposite sides of said flow channel.

2. A plasma accelerator as defined in claim 1 wherein each of saidelectrode pairs consists of a cathode and an anode, and said electricalcircuit includes: a unidirectional potential source connected acrosseach cathode-anode pair for providing a potential gradient normal tosaid linear axis, and a unidirectional potential source con nectedbetween each am'ally spaced cathode-anode pair for providing an axialpotential gradient.

3. A steady flow, linear, high-density plasma accelerator comprising: aflow channel having a longitudinal axis, a plurality of positiveelectrodes spaced along said longitudinal axis and comprising a firstwall of said channel, a plurality of negative electrodes spaced alongsaid longitudinal axis and comprising a second channel wall opposingsaid first wall, a pair of elongated insulating strips extendingparallel to said longitudinal axis and positioned between said positiveand negative electrodes, said strips comprising opposing third andfourth walls of said channel, an electrical circuit interconnecting saidpositive and negative electrodes, said circuit being adapted to providean electric field across said channel directed at a slant angle to saidlongitudinal axis, and means, positioned on opposite sides of said flowchannel, for providing a magnetic field across said channel directedperpendicular to said electric field.

4. A plasma accelerator as defined in claim 3, wherein said electricalcircuit includes: unidirectional sources of potential connected acrossopposing pairs of positive and negative electrodes for providing apotential gradient perpendicular to said longitudinal axis, and aunidirectional potential source connected between each axially spacedpair of positive and negative electrodes for providing an axialpotential gradient.

5. A plasma accelerator as defined in claim 3, wherein said negativeelectrodes comprise thermionic emitters adapted to be heated to electronemission by the plasma flow through the channel.

6. A plasma accelerator as defined in claim 3, wherein an insulatingspacer member is positioned between each of said plurality of positiveelectrodes and between each of said plurality of negative electrodes.

7. A steady flow, plasma acelerator comprising: a plurality ofcathode-anode pairs spaced along a linear axis, insulating memberspositioned between and separating said cathode-anode pairs, saidcathode-anode pairs defining a how channel along said linear axis, thecathodes of each of said pairs consisting of a material having a highrate or" electron emissivity, an electrical circuit interconnecting saidcathode-anode pairs, said electrical circuit including: a unidirectionalpotential source connected across each cathode-anode pair for providinga potential gradient normal to said linear axis, and a unidirectionalpotential source connected between each axially spaced cathodeanode pairfor providing an axial potential gradient; and means for providing amagnetic field across said channel directed perpendicular to theelectric field created by said vertical and axial potential gradients,said means being positioned adjacent said flow channel.

8. A plasma accelerator as defined in claim 7, wherein said cathodematerial is lanthanum hexaboride.

9. A plasma accelerator as defined in claim 7, wherein said cathodematerial is pure tungsten.

10. A plasma accelerator as defined in claim 7, wherein said cathodematerial is thoriated tungsten.

3. A STEADY FLOW, LINEAR, HIGH-DENSITY PLASMA ACCELERATOR COMPRISING: AFLOW CHANNEL HAVING A LONGITUDINAL AXIS, A PLURALITY OF POSITIVEELECTRODES SPACED ALONG SAID LONGITUDINAL AXIS AND COMPRISING A FIRSTWALL OF SAID CHANNEL, A PLURALITY OF NEGATIVE ELECTRODES SPACED ALONGSAID LONGITUDINAL AXIS AND COMPRISING A SECOND CHANNEL WALL OPPOSINGSAID FIRST WALL, A PAIR OF ELONGATED INSULATING STRIPS EXTENDINGPARALLEL TO SAID LONGITUDINAL AXIS AND POSITIONED BETWEEN SAID POSITIVEAND NEGATIVE ELECTRODES, SAID STRIPS COMPRISING OPPOSING THIRD ANDFOURTH WALLS OF SAID CHANNEL, AN ELECTRICAL CIRCUIT INTERCONNECTING SAIDPOSITIVE AND NEGATIVE ELECTRODES, SAID CIRCUIT BEING ADAPTED TO PROVIDEAN ELECTRIC FIELD ACROSS SAID CHANNEL DIRECTED AT A SLANT ANGLE TO SAIDLONGITUDINAL AXIS, AND MEANS, POSITIONED ON OPPOSITE SIDES OF SAID FLOWCHANNEL, FOR PROVIDING A MAGNETIC FIELD ACROSS SAID CHANNEL DIRECTEDPERPENDICULAR TO SAID ELECTIC FIELD.